Cells of macrophage lineage are the primary cells in which HTV replicates in the CNS. Macrophages increase inflammatory reactions and promote neurodegeneration. Accumulation of activated macrophages in infected brains, rather than the level of virus replication, is thought to be the best correlate of MTV-associated dementia. Bone marrow dysfunction is another feature of HIV disease and it may lead to alterations in the differentiation cascade and maturation of monocytic lineage. Whether the increase in activated macrophages in the brain is a result of trafficking of unique monocytic cell subsets produced by bone marrow during the late stages of infection is the subject of this proposal. Intriguingly, two different groups have provided evidence that a CD14l(low), CD16(high) and CD 14+ CD69+ monocyte subsets emerge during HIV infection and that these cells correlate with cognitive impairment and HIV dementia. Using our well-characterized SIV/macaque animal model of HIV neuropathogenesis, in which SIV encephalitis correlates with accumulation/activation of macrophages, selective replication of neurovirulent genotypes and increase caspase-3 activity in the brain, and in which preliminary data have shown that bone marrow was a virus reservoir for the neurovirulent genotype during acute infection, we hypothesize that HIV/SIV infection alters macrophage differentiation in bone marrow, which results in the production of hyperactive neuroinflammatory monocyte/macrophage population that readily traffic to the brain and releases neurotoxic factors. We further hypothesize that combination antiretroviral drug treatment (CART; Nelfinavir + Tenofovir) may ameliorate neuroinflammation by restoring bone marrow hematopoiesis. Because of the potential use of bone marrow transplantation combined with gene therapy in HTV vaccine strategy, Aim 1 will address the question of whether hematopoietic stem cells are infected and to what extent they are a virus reservoir. Aim 1 will also examine the effect of SIV infection on the hematopoietic cell pool and whether CART reverses these effects. Based on others' and our preliminary data, Aim 2 will compare expansion of the CD14low CD16high and CD 14+ CD69+ monocyte subsets in the blood of SIV infected, untreated and SIV-infected, CART-treated macaques. This Aim will also examine CD14low CD16high and CD 14+ CD69+ monocyte subsets purified from acutely and terminally infected brains and bone marrow for their capacity to produce soluble factors that induce apoptosis in primary human neuronal cultures. We will also identify these neurotoxic factors in culture supernatants of these cell subsets and in the brain and bone marrow of SIV infected macaques.